A number of afferent and efferent projections of the cerebellar cortex have been shown to terminate as a series of parasagittally oriented zones. The concept of a "cerebellar module" or "microzone" has been proposed as being the basic functional unit of the cerebellum which modulates a certain aspect of motor behavior. The experiments in this proposal are designed to test hypotheses relating to the concept of a "cerebellar module" and to discern the organizational principles of brainstem afferents to the posterior cerebellar cortex in the rat and mouse. These experiments are as follows: 1. Inject individual brainstem nuclei with tritiated amino acids to determine the organization of their inputs to the cerebellar cortex. 2. Inject wheat germ agglutinin-horseradish peroxidase (WGA-HRP) conjugate into different mediolateral positions in the same and different cerebellar lobules to map the location, of brainstem cells which project to these specific regions. 3. Inject to retrograde tracers into two predetermined parasagital zones to determine whether individual neurons project oxon collaterals to more than one zone. 4. Inject WGA-HRP in the cerebellar cortex and tritiated amino acids in the brainstem of the same animal to corroborate the results of the studies listed above. 5. Inject two retrograde tracers, one in cerebellar cortex and one in the deep cerebellar nuclei to determine if olivary cells project collaterals onto deep nuclear cells which receive afferents from the Purkinje cells innervated by the same group of olivary cells. The careful execution of the experiments should provide interesting and exciting information concerning the anotomical organization of afferent input to the posterior cerebellar cortex. It should also provide some important data upon which we can evaluate the concept of the "cerebellar module" as the basic functional unit of the cerebellum and some information concerning the nature of that function. Hopefully this information will lead to a clearer understanding of the cerebllum's role in modulating motor activity in the normal animal and in animals with cerebellar or other motor diseases.